The proposed research may be subdivided into two areas of investigation, both of which utilize the goldfish Mauthner cell (M-cell) as a model. In the first aspect, properties of the following identified synapses will be studied with simultaneous intracellular recordings from pre- and postsynaptic elements: 1) the inhibitory synapse between identified interneurons and the M-cell, and 2) the mixed electrotonic and chemical excitatory synapses between vestibular afferents and the M-cells. Emphasis will be placed upon analysis of mechanisms of transmitter release, input-output relations, and frequency characteristics of the synapses. At the mixed synapses, the coupling coefficients and possible regulatory interactions between the chemical and electrotonic modes of transmission will be studied. Also, the patterns of M-cell collateral and sensory inputs to the inhibitory interneurons will be determined in order to develop a model of interneuron function in reflex control. The second aspect of the proposal deals with the axon reaction of central neurons. Intra- and extracellular recordings will be used to determine alterations in M-cell membrane properties and synaptic interactions produced by spinal cord transection. These studies will be correlated with morphological investigation at the light and electron microscopic levels. Finally, the role of inter- and intraneuronal trophic interactions in the maintenance and regulation of M-cell membrane properties and synaptic transmission will be studied. These investigations will involve the intracellular injection of labeled amino acids, nucleoside derivatives, glycoproteins and mucopolysaccharides, and their disposition will be determined with the techniques of autoradiography. Emphasis will be placed on an analysis of dendritic transport functions. These systems will also be investigated after axotomy and intracellular injections of colchicine, which blocks axonal and dendritic flow, in order to correlate the trophic functions with neuronal and trans-neuronal responses to injury.